Heat Pump COP Explained — Real London Numbers

COP (Coefficient of Performance) is the instantaneous ratio of heat output to electrical input. A COP of 3 means 3 kW heat for 1 kW electricity. Measured at specific conditions (typically 7°C outside, 35°C flow).

SCOP (Seasonal COP) is the average COP over a whole heating season, weighted by typical outdoor temperatures. This is the number that determines your annual bill.

Nameplate COP is often 4.5+. Real SCOP for a London install in 2026 is typically 2.8-3.6 depending on system design.

A 3.2 SCOP installation uses 1 kWh of electricity to deliver 3.2 kWh of heat over a year. That's the metric to compare with a gas boiler (which is typically 0.85-0.92 efficient).

Air source heat pump (ASHP), 35°C flow, underfloor heating, well-insulated property — 3.6-4.0 SCOP. Best case.

ASHP, 45°C flow, oversized radiators, post-2010 property — 3.2-3.6 SCOP.

ASHP, 55°C flow, standard radiators, period property — 2.6-3.0 SCOP.

Ground source heat pump (GSHP), any flow — typically 3.8-4.5 SCOP because ground source is more stable.

Most London retrofits land at 2.9-3.4 SCOP. Plan for this when calculating bills, not the manufacturer's 4.5 nameplate.

Below 0°C outdoor, ASHP COP drops to 2.0-2.5. London hits this 15-25 days a typical year.

Around 7°C (typical London winter day) — COP 3.0-3.5.

Above 12°C (autumn/spring) — COP 3.5-4.5.

The weighted average — SCOP — assumes the typical London winter temperature curve. Bad winter, lower SCOP. Mild winter, higher SCOP.

Cold-weather backup matters. Some ASHPs include immersion-element backup for sub-zero days; COP for that hour drops to 1.0. Plan for 5-10% of annual heat from backup.

A typical 3-bed London semi using 12,000 kWh of heat per year (gas equivalent).

Gas boiler at 90% efficiency: 13,300 kWh of gas at 6p/kWh = £798 per year (plus standing charge).

ASHP at SCOP 3.2: 3,750 kWh of electricity. On standard tariff (27p): £1,012 — more expensive than gas.

Same ASHP on Cosy tariff (60% shifted to 13p, 40% at 25p): 3,750 kWh × 0.6 × 13p + 3,750 × 0.4 × 25p = £668. Cheaper than gas.

Same ASHP plus solar (40% of heating from solar at zero marginal cost) on Cosy: £400. Half the gas cost.

Oversize the heat emitters. Radiator sizing for 45°C flow not 60°C. Underfloor wins; oversized radiators in priority rooms (living, master bedroom) are next best.

Insulate first. Heat pumps work best in well-insulated properties. Loft, cavity wall, and good draught-proofing all push SCOP up by 0.2-0.4.

Right-size the heat pump. Oversized heat pumps short-cycle and drop SCOP. MCS-installer heat loss calc should be the basis.

Smart controls. Weather compensation and load-following both push SCOP up. Boost mode used only when needed.

If your property is post-2000, well-insulated, and you can do oversized radiators — ASHP at SCOP 3.4 with BUS grant (£7,500) and 0% VAT until 1 May 2027 is a clear win.

If your property is pre-1980 with poor insulation — ASHP at SCOP 2.6 is marginal vs gas on standard tariff. Insulate first, then heat pump.

If you can't insulate (listed building, conservation area) — consider hybrid heat pump (ASHP + gas) until full retrofit is possible. Or wait for hydrogen-blend gas (post-2030 likely).

MEES context: heat pump alone rarely hits band C — it must be paired with insulation. Plan both as one project before the 1 October 2030 deadline.